Field vs Plan: Understanding the Two Probability Modes

Lumina gives you two different aurora probability numbers, and they serve two different purposes. Here's what each one means and when you should pay attention to it.

The two modes

Think of it like checking the weather before heading out. There's the "what's happening right now?" check — is it raining at this moment? Then there's the "should I pack an umbrella later?" check — what's the forecast for tonight?

Lumina's Field mode is the "right now" check. Plan mode is the "later tonight" check.

Field Mode — "Right now" (10–15 minutes)

Field mode gives you the short-term odds . It uses arrival-shifted IMF data — meaning Lumina estimates how long the solar wind took to travel from the satellite (parked about 1.5 million km upstream of Earth) to actually reach our magnetosphere. That way the B_z reading you see is as close as possible to what's really hitting Earth right now.

This is the mode you check when you're already outside, or wondering if you should duck out for a quick look. It reacts fast to southward turnings of the IMF — if B_z suddenly flips negative, Field mode odds will climb within minutes.

Field mode doesn't apply the visibility factor — it tells you what's happening in the magnetosphere regardless of whether the oval is above your horizon. That's intentional: if you're at a higher latitude or have a clear southern horizon, you want the raw signal.

Plan Mode — "Later tonight" (30–90 minutes)

Plan mode is your planning horizon . It uses raw L1 data (no arrival shift) plus sustained forcing metrics — it's less jumpy because it cares about what the solar wind has been doing over the last hour, not just the last few minutes.

The key ingredient in Plan mode is storm memory . When the magnetosphere gets a sustained beating from southward IMF, it charges up like a battery. Plan mode tracks this charge state:

• Charges fast — about 90 minutes of sustained southward B_z to reach full memory.
• Decays slowly — about 4 hours to fully discharge after the driving stops.

This means Plan mode won't drop to zero during a brief lull in the solar wind. If there's been good activity and B_z temporarily swings north, Plan mode carries forward some of that built-up energy. That's realistic — the magnetotail doesn't instantly drain just because B_z flicked positive for ten minutes.

Plan mode does apply the visibility factor from oval geometry — so if the auroral oval is below your horizon, Plan odds get scaled down to reflect that geometric reality.

Which mode when?

Reading the sparklines

On the main dashboard card you'll see two tiny sparkline graphs tracing the last 3 hours of probability. The Field line tends to be more jagged (responding to real-time IMF swings) while the Plan line is smoother (carrying storm memory). When they diverge — Field spikes but Plan doesn't follow — that usually means a brief southward pulse that probably won't sustain. When both lines climb together, that's when you should start seriously considering a trip outside.

Technical summary